Process for treating cellulosic digesting liquor containing soap

ABSTRACT

PROCESS FOR TREATING A CELLULOSIC DIGESTING LIQUOR CONTAINING SOAP WITH A LIQUID HYDROCARBON AND ALCOHOL TO SEPARATE THE SOAP FROM THE LIQUOR.

United States Patent 3,578,628 PROCESS FOR TREATING CELLULOSIC DIGEST-ING LIQUOR CONTAINING SOAP Sun W. Chun, Pittsburgh, Pa., and William H.Friend, Edison, N.J., assignors to Union Camp Corporation, New York,N.Y. No Drawing. Filed Apr. 21, 1969, Ser. No. 818,049

Int. Cl. C095? 1/00 US. Cl. 260-975 3 Claims ABSTRACT OF THE DISCLOSUREProcess for treating a cellulosic digesting liquor containing soap witha liquid hydrocarbon and alcohol to separate the soap from the liquor.

BACKGROUND OF THE INVENTION In the past, cellulosic digesting liquor,which is the used cooking liquor from pulp digesters, has beenconcentrated by various evaporating techniques. In a typical case thisdigesting liquor is the used black cooking liquor from kraft digesters.Kraft digesters are employed to process wood pulp. This black liquorcontains many valuable by-products which are commonly separated from theused cooking liquor by a multiple evaporation procedure.

Among the valuable components of black liquor are various tall-oilsoaps. Heretofore, such soap has been separated from the black liquor byfeeding concentrated liquor, also known as skimmer feed, into a skimmingtank. The liquor is retained in the skimming tank until the entrainedtall-oil soap floats to the surface of the liquor, where it may beremoved by skimming. The skimmed soap is the raw material forconventional tall-oil processing.

The black liquor is discharged from the bottom of the skimming tankafterwards and fed to the next evaporator elfect. Black liquor is atthis point commonly known as skimmer discharge. This liquor containsabout 25% of the soap which was present in the skimmer feed. Thereforethe soap recovery efficiency of present methods is about 75%. Generally,the remaining soap in the skimmer discharge is lost to the process.

In an ellort to increase the quantity of soap recovered from skimmerfeed, it has been proposed to inject a solvent, usually a liquidhydrocarbon, into the skimmer feed, prior to conventional skimming. Thesolvent flocculates the entrained soap in skimmer feed such that asoap-solvent layer rises to the top of the skimmer tank and isrecovered. The recovered soap solvent layer is processed in order toremove solvent and recover soap. The skimmer discharge, containingresidual soap and solvent, is fed to the next evaporator effect wherethe solvent is distilled and separated from the black liquor.

This process has several major drawbacks. Solvent recovery from theskimmer discharge has proven highly unsatisfactory. This low solventrecovery is due, in part, to the greater miscibility of the typicalhydrocarbon solvent in black liquor than would have been expected. Also,there appears to be a definite limit as to the quantity of soap whichmay be recovered from the skimmer feed by this process. This limit isdetermined, in part, by the quantity of soap which is dissolved in theskimmer feed, as opposed to the soap present in skimmer feed as discrete soap particles. The hydrocarbon solvent is relatively inelfectivein extracting dissolved soap from black liquor, and acts generally as acoascervating agent for soap particles.

Patented May 11, 1971 It is, therefore, a principal object of thisinvention to provide a process for removing soap from a cellulosicdigesting liquor employing a solvent which is readily recovered fromsaid liquor, for recycling to the process. It is another object of thisinvention to provide a process for recovering both dissolved soap andsoap particles from black liquor. It is an additional object of thisinvention to provide a soap removal process which is readily integratedwith conventional black liquor concentrating processes.

Other aspects, objects and advantages of this invention will be evidentas the discussion proceeds.

The above and other objects of this invention are achieved by treating acellulosic digesting liquor containing soap with a liquid hydrocarbonand an alcohol. The liquid hydrocarbon and alcohol (hereinafter referredto as mixed solvent) collect the soap in a phase or layer at the surfaceof the black liquor. This layer, containing, in part, soap, liquidhydrocarbon, alcohol, and water is removed from the black liquor and maybe treated so as to recover the hydrocarbon and alcohol for recyclepuroses. P De-soaped black liquor containing residual mixed solvent isrecycled to conventional black liquor concentrating processes. Mixedsolvent is recovered from the concentrating process by conventionaltechniques and recycled for further use.

The novel process is employed, for example, to extract tall oil soapfrom the sulfate black liquor employed as the cooking liquor in kraftdigesters. As black liquor is concentrated by evaporation inmultiple-effect evaporator, the concentration of soap in the liquorincreases. When the black liquor concentration is from about 22 to 26%by weight solids based on total weight of liquor, the solubility of theentrained soap reaches a minimum. At this point in the concentratingprocess, the black liquor (skimmer feed) flows into a conventionalskimming tank and is retained therein for about an hour. During thisperiod, soap fioats to the surface to be skimmed. After skimming, theblack liquor is discharged from the bottom of the skimming tank. At thispoint the black liquor is commonly known as skimmer discharge.

Additional tall oil soap is processed from said skimmer discharge bymixing the skimmer discharge with the mixed solvent and then introducingthis mixture into a holding tank. Upon standing, a layer comprisingsoap, water, and mixed solvent separates and rises to the surface of thetank.

This layer may be separated from the black liquor remaining byconventional separation techniques, such as skimming, decanting,pumping, or the like. Mixed solvent is separated from the soap byconventional techniques. If desired, the mixed solvent may be evaporatedfrom the soap by steam stripping, flash drying, vacuum distillation, orthe like. The recovered mixed solvent is recycled to treat additionalskimmer discharge.

The black liquor remaining in the holding tank after the soap layer hasbeen removed is discharged from the holding tank and may be introducedinto an appropriate stage of the multi-effect evaporator. Thisdischarged liquor contains minute amounts of residual mixed solvent.This solvent is readily recovered from the liquor in the multiple-effectevaporator.

The liquid hydrocarbon constituent of the mixed solvent is aliphatic oraromatic. The hydrocarbon may be saturated or unsaturated. Typicalexamples of hydrocarbons employed are: alkanes, alkenes, arylalkanes,cycloalkanes, cycloalkenes, and mixtures thereof. Generally 3 the liquidhydrocarbon employed has a boiling point in the range of from about 100to 200 C.

Examples of specific hydrocarbons includes heptane, octane, decane,2-ethyl heptane, dodecane, pentane, cyclooctane, cyclohexane,cyclohexene, norcarane, cis or trans Decalin, diethylbenzene,cyclohexylbenzene, and the like, and mixtures thereof.

Generally the alcohol employed in the process can be aliphatic oraromatic. The alcohol may be saturated or unsaturated. Examples oftypical alcohols include alkanols, alkenols, alicyclic alcohols, alkarylalcohols, aralkyl alcohols and the like.

Specific examples of alcohols employed in the process include: methanol,ethanol, isopropanol, cyclopentanol,

Z-ethyl 1 butanol, tert-arnyl alcohol, benzyl alcohol, allyl alcohol,methylcyclohexanol, furfuryl alcohol, ethylphenylcarbinol,methylphenylcarbinol, cyclohexylcarbinol, metatolylcarbinol, and thelike.

Alcohols having functional groups are also employed. Examples of suchalcohols are: 2bromo-ethanol, 23- dich1oro1-propanol, and the like.Generally, the functional groups on said alcohol must not react with anyconstituent in the black liquor.

The choice of alcohol for a particular system depends, in part, on thequantity of water in the black liquor, the temperature of the blackliquor, the chemical composition of the black liquor, and also thechoice of hydrocarbon. Usually, the alcohol should be of sufficientlyhigh volatility as to be readily separable from the soap layer byevaporating techniques. Generally alcohols with high water solubilityare not employed in order to reduce solvent loss.

Typical mixtures of alcohol and hydrocarbon include the following:

( l) Propenylbenzene, cyclopentanol (2) Undecane, 4-me'thyl-2-pentanol(3) Benzene, 2-phenyl-l-heptanol (4) 1, 3, S-triethylbenzene, benzylalcohol Other mixtures are formed by combining any alcohol listed abovewith any hydrocarbon listed above.

By employing the aforementioned process, it has been found that certainadvantages are achieved. The quantity of residual mixed solventdischarged with black liquor into the multiple-effect evaporator issignificantly reduced, as compared to prior art desoaping processes.Further, the quantity of mixed solvent lost to the black liquor, afterevaporation or the like, is less than one tenth of the quantity ofhydrocarbon solvent lost by previous processes. Further, soap extractionis improved, owing to the enhanced solubility of entrained soap in mixedsolvent.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Generally, enhanced soaprecovery is achieved and reduced solvent loss is observed, when thenovel process is employed in conjunction with a conventional skimmerfeed skimming process. The mixed solvent of the invention is preferablyadded to skimmer discharge to recover incremental tall oil soap, afterconventional skimming.

The process is preferably employed by premixing the alcohol andhydrocarbon and thereafter adding the premixed solvent to skimmerdischarge. The mixed solventskimmer discharge mixture is preferablyagitated with aeration to free entrained soap particles. After thoroughmixing, the mixture is preferably transferred to a holding tank. In theholding tank a soap phase forms at the top of the mixture. Generally,the soap phase is sufiiciently formed so as to be easily removed afterat least about 30 minutes of settling. For maximum soap recovery it ispreferred to settle the contents in the holding tank for from about oneto two hours.

It is preferred to employ a covered holding tank in order to minimizesolvent loss. The soap phase in the holding tank is skimmed from thesurface of the black liquor and transferred to a mixed solvent removalstage, preferably a steam stripper. The entire process is suitablyemployed on a continuous or semi-continuous basis, although it may beperformed on a batch-Wise basis.

The hydrocarbon is preferably immiscible With black liquor to reducesolubility losses. Preferred hydrocarbons include aromatic hydrocarbonsor mixtures of aromatic hydrocarbons. Enhanced results are obtained byaromatics having an initial boiling point in the range of from about 300to 400 F. and flash points of at least about 100 F. Commercial aromaticmixtures boiling in this range which are especially preferred includeSolvesso 100, Solvesso 150, and Neville 250W. Xylene is an excellenthydrocarbon for the purposes of this invention. Examples of otherpreferred aromatic hydrocarbons include, for instance: toluene,ethylbenzene, xylene, isopropylbenzene, and mixtures thereof.

Preferred hydrocarbons also include petroleum solvents boiling in thenaphtha range. Examples of still further preferred hydocarbons arecycloalkenes; including pinene, camphene, bornylene, carene, and otherturpentine constituents. Sulfate turpentine which may be crude ordistilled and comprises primarily cycloalkenes, is also employed.

Monohydric alcohols are preferred, especially alkanols. Particularlypreferred alcohols are those alkanols having from about 8 to about 12carbon atoms. Such alcohols include: octyl alcohol, nonyl alcohol,undecyl alcohol, decyl alcohol, and dodecyl alcohol.

The alcohol is preferably immiscible with black liquor and insoluble inwater to reduce solubility losses. A most preferred alcohol is decylalcohol.

Typical examples of preferred mixed solvents include:

(1) Xylene; methylnonyl alcohol;

(2) Solvesso 150 (an aromatic hydrocarbon solvent flash point 150 F.,and derived from petroleum); decyl alcohol;

(3) Sulfate turpentine (a mixture of cycloaliphatics, predominantlypinene and derived from wood pulp); decyl alcohol;

(4) Solvesso 100 (an aromatic hydrocarbon solvent flash point 100 F. andderived from petroleum); isodecyl alcohol;

(5) Pinene, n-decyl alcohol;

(6) Ethylbenzene, isononyl alcohol;

(7) Toluene, n-undecyl alcohol;

(8) Isopropyl benzene, n-dodecyl alcohol;

(9) Benzene-toluene mixture, decyl alcohol;

(10) Sulfate turpentine (distilled) isodecyl alcohol.

Generally, the proportions of alcohol to hydrocarbon may vary broadly.It is preferred that the proportion of alcohol, particularly monohydricalkanols, having from about 8 to 12 carbon atoms, is from about 5 to byweight of the total mixed solvent and the proportion of hydrocarbon,particularly aromatic hydrocarbon having an initial boiling point in therange of from about 300 to about 400 F. is from about to 25% by weightof the total mixed solvent.

Enhanced results are obtained and accordingly, it is preferred to employthe mixed solvent in the following proportions: 10% to 50% by weightalcohol and 90% to 50% by weight hydrocarbon, based on the total weightof alcohol and hydrocarbon.

If desired, to aid in the extraction of soap, conventional electrolytesmay also be employed. These are added to the skim feed or the skimdischarge.

The following examples are illustrative only.

EXAMPLE I In order to demonstrate the effect of the novel process onsoap recovery, conventionally skimmed back liquor was treated withmixtures of SOlvesso and n-decyl alcohol. Solvesso 150 is a proprietarymixture of aromatic solvents derived from petroleum having an initialboiling point of about 360 F. and a flash point of 150 F.

A metered amount of skimmed discharge containing 0.6% by weight tall oilon solids and a metered amount of mixed solvent were mixed in a Venturitube. The mixture was introduced into a mixing vessel, said vessel beingmounted in a settling tank. The mixture was stirred and aerated therein.After about five minutes of stirring, the mixture overflowed to thesettling tank, where it was accumulated. An outlet valve on the bottomof the settling tank was opened, such that the level of material in thesettling tank remained constant. After an hour, samples were taken fromthe black liquor discharge. A second set of samples was taken at ahigher mixed solvent rate. The samples were analyzed for residualtall-oil in the black liquor phase.

The residual tall-oil in the black liquor was analyzed by Mill TechnicalDivision Method A.M. 265, commonly known as the Buckeye Method asdescribed in TAPPI, vol. 42, 1959. The concentrations of tall-oil in thetable below are given as the weight percent tall-oil based on totalliquor solids. In the table below, the proportion of alcohol is given inweight percent of the total mixed solvent. The solvent flow rate isexpressed in volume percent of mixed solvent per volume of black liquor.

RESIDUAL TALL-OIL ANALYSES The results demonstrate that the mixedsolvent comprising an alcohol and liquid hydrocarbon, effectivelyextracts soap from cellulosic digesting fluid, particularly kraft blackliquor. The level of tall-oil on ,black liquor solids initially presentin the skimmer discharge prior to treatment was 0.60 Weight percenttall-oil on solids. A marked increase in tall-oil recovered is shown bythe addition of mixed solvent to skimmer discharged.

When other liquid hydrocarbons and alcohols are substituted for themixed solvent employed in the previous test, similar results areobtained. Other mixed solvents which may be substituted for the aboveinclude:

( 1) Nonyl alcohol, xylene (2) Undecyl alcohol, turpentine (3) Ethyleneglycol monomethyl ether, toluene and mixtures thereof.

EXAMPLE H In order to demonstrate the efficiency with which the mixedsolvent is recovered from both the black liquor and soap phases, blackliquor was treated according to the procedure of Example I, employing amixed solvent, n-decyl alcohol and Solvesso 150, and soap skimmedtherefrom. The treated black liquor discharge was then concentrated byevaporation from its initial solids content of about 25% to a finalcontent of 29% solids.

The skimmed soap phase was steam-stripped to recover solvent. The mixedsolvent recovered from both the discharged black liquor and thesteam-stripper was individually analyzed. To provide comparativeresults, a run was performed according to the above procedure employingsolely a hydrocarbon solvent. The following table shows the resultsobtained.

The treated skimmer discharge columns show the quantity of mixed solventin parts per million present in black liquor recycled to themultiple-effect evaporators after soap extraction with mixed solvent.The total mixed solvent loss represents the mixed solvent lost to blackliquor which has been treated to evaporate mixed solvent, and the mixedsolvent lost to the soap phase recovered from the steam-stripper. Thesolvent flow rate and percent decyl alcohol columns were reported as inExample I.

SOLVENT ANALYSES Solvent Mixed solvent, flow Treated skimmer dischargepercent by rate in Total mixed weight decyl volume Hydrocarbon Alcoholsolvent loss alcohol percent in p,p.m. 1n p.p.m in p.p.m.

The results demonstrate the reduced solvent loss of the mixed solvent ascompared to a simple hydrocarbon solvent. The results also demonstratethe enhanced separa tion of the novel solvent from the black liquorskimmer discharge which is treated in the holding tank to removeadditional tall-oil soap. Similar results are obtained when other mixedsolvents are substituted for the above solvent mixture.

The additional (incremental) soap recovered by the novel process wascompared with normal skim soaps, as presently obtained duringconventional skimming procedures, without employing solvents. Theincremental soap showed a greater than 50% reduction in unsaponificablematter as compared to normal skim soap. Other advantages are obtained bythe invention. Whereas conventional skimming without solvents generallycauses an undesirable, hard to process, stable foam to be produced atthe surface of the black liquor, it has been found that an easilyprocessable thick skim is produced by the action of the mixed solvent.

We claim:

1. A process for treating a cellulosic digesting liquor containing soapcomprising admixing said liquor and a mixed solvent comprising a mixtureof from about 90 to 50% by weight petroleum aromatic hydrocarbons havingan initial boiling point in the range of from about 300- 400 F. and aflash point of at least about F. and from about 10 to 50% by weightdecyl alcohol, wherein said weights are based on the total weight of themixed solvent, whereby a soap-containing phase is formed at the surfaceof the mixture which may be removed and soap recovered therefrom.

2. A process for treating kraft black liquor skimmer dischargecontaining tall-oil soap comprising:

(a) forming a liquid mixed solvent comprising a mixture of from about 90to 50% by weight petroleum aromatic hydrocarbons having an initialboiling point in the range of from about 300400 F. and a flash point ofat least about 100 F. and from about 10 to 50% by weight decyl alcohol,wherein said weights are based on the total weight of the mixed solvent;

(b) mixing said solvent and said skimmer discharge;

(c) introducing said mixture into a holding tank, wherein a layercomprising soap and mixed solvent is formed;

(d) separating said layer from the black liquor; and

(e) recovering soap from said layer.

3. In the process for concentrating black liquor containing soap fromkraft digesters by the stepwise evaporation of volatile componentstherefrom, wherein a portion of the soap is separated from said blackliquor by settling said liquor, skimming off a soap layer formed on thesurface of said liquor and recycling the remaining black liquor skimmerdischarge to the concentrating process, the improvement by means ofwhich additional soap is separated from the skimmer discharge whichcomprises:

(a) mixing skimmer discharge with a mixed solvent comprising a mixtureof from about 90 to 50% by weight petroleum aromatic hydrocarbons havingan initial boiling point in the range of from about 300- 400" F. and aflash point of at least 100 F. and from about 10 to 50% by weight decylalcohol, wherein said weights are based on the total weight of the mixedsolvent; and

7 (b) introducing said mixture into a tank whereby a soap containingphase rises to the surface of said mixture.

References Cited UNITED STATES PATENTS 2,399,588 4/1946 Weston et a1.26097.5 2,519,903 8/1950 Hasselstrom 26097.7 2,530,809 11/1950Christensen et a]. 260--97.7

8, 2,717,890 9/1955 Drechsel 260-965 2,866,739 12/1958 Ciesielski et a1.26097.7 3,449,313 6/1969 Bolger et a1 260-975 DONALD E. CZAJA, PrimaryExaminer 5 R. W. GRIFFIN, Assistant Examiner

